Aromatic hydrocarbon compounds such as benzene are frequently used for producing transportation fuels and petrochemicals such as styrene, phenol, nylon, polyurethanes and many others. Benzene can be produced, e.g., by steam cracking and naphtha reforming. During steam cracking, a C2+ hydrocarbon feed reacts in the presence of steam under high-temperature pyrolysis conditions to produce a product comprising molecular hydrogen, C4− olefin, other C4− hydrocarbon, and C5+ hydrocarbon including aromatic hydrocarbon. The yield of aromatic hydrocarbon from steam cracking is generally much less than the yield of light hydrocarbon, and processes of significant complexity are typically needed for aromatics separation and recovery. Naphtha reforming catalytically produces a product having a much greater content of aromatic hydrocarbon than does steam cracker effluent, but the naphtha feed is itself useful for other purposes such as motor gasoline blendstock.
Attempts have been made to overcome these difficulties, and provide an efficient process for producing aromatic hydrocarbon at high yield from a relatively inexpensive feed. For example, processes have been developed for producing light aromatic hydrocarbon (e.g., benzene, toluene, and xylenes—“BTX”) from paraffinic C1-C4 feeds. The processes typically utilize a catalyst having a molecular sieve component e.g., ZSM-5, and a dehydrogenation component, such as one or more of Pt, Ga, Zn, and Mo. These conventional processes typically operate at high temperature and low pressure. Although these conditions increase the yield of aromatic hydrocarbon, they also lead to an increased rate of catalyst deactivation, mainly resulting from increased catalyst coking.
Reverse-flow reactors can be used to lessen the amount of catalyst coking. The reactor carries out a catalytic reaction such as hydrocarbon aromatization during forward flow, which deposits coke proximate to the catalyst. Following the forward flow reaction, a reverse-flow oxidation reaction combusts at least a portion of the accumulated coke. For example, U.S. Pat. No. 4,704,497 discloses carrying out catalytic dehydrogenation of a hydrocarbon feed in forward-flow, and then removing coke deposits on the dehydrogenation catalysts by conveying an oxygen-containing gas through the reactor in reverse-flow. Since at least a portion of any accumulated coke is removed from the catalyst by oxidation during reverse-flow, the long-term rate of catalyst coke accumulation can be controlled.
A more recent process, described in U.S. Pat. No. 8,754,276, includes carrying out a catalytic dehydrogenation reaction in a reaction zone of a reverse-flow reactor. The catalytic dehydrogenation is operated in forward flow to produce unsaturated products such as olefin and aromatic hydrocarbon (reaction mode). The reaction zone has a lesser temperature at the upstream end of the reaction zone. The temperature profile increases monotonically across the reaction zone to a greater temperature at the downstream end, with upstream and downstream being with respect to the flow of hydrocarbon feed. This temperature profile is said to benefit the catalytic dehydrogenation reaction by lessening undesired reversion reactions which produce products of greater saturation. Since the dehydrogenation reaction is endothermic, the reaction zone cools during dehydrogenation mode, which eventually lessens dehydrogenation efficiency. The reaction zone is reheated by operating the reactor in regeneration mode. During regeneration mode, a combustion mixture comprising oxidant and fuel is conveyed to the reactor. The fuel is combusted with the oxidant in a combustion zone located within the reactor. The combustion zone contains a selective combustion catalyst, and is located upstream of the reaction zone with respect to the flow of the combustion mixture. Heat transferred from the hot combustion products to the reaction zone reheats the reaction zone to a temperature sufficient for carrying out dehydrogenation mode operation. After the reactor is sufficiently reheated, regeneration mode is halted, and reaction mode operation is re-commenced. According to the patent, placing the selective combustion catalyst in the combustion zone ensures that combustion of the combustion mixture is carried out in the combustion zone, not in the reaction zone. Carrying out combustion within the reaction zone is said to be undesirable because it results in a deviation from the desired monotonic temperature profile, which during reaction mode would increase the undesirable reversion of product olefin and aromatic hydrocarbon to more saturated molecules. However, decreasing the amount of oxidant proximate to the dehydrogenation catalyst during the combustion mode also lessens the amount of catalyst coke that can be removed from the dehydrogenation catalyst.
Another way to control catalyst coking involves carrying out the aromatization processes with a decreased selectivity for catalyst coke. For example, U.S. Pat. No. 4,855,522 discloses using a dehydrocyclization catalyst comprising (a) an aluminosilicate having a silica:
alumina molar ratio of at least 5 and (b) a compound of (i) Ga and (ii) at least one rare earth metal. The aromatization is carried out at a temperature ≥450° C. (e.g., 475° C. to 650° C.) and a pressure of from 1 bar to 20 bar. Other processes limit selectivity for catalyst coke by carrying out the reaction for a relatively short time (e.g., less than a day), and then halting the reaction so that the catalyst can be regenerated. For example, U.S. Patent Application Publication No. 2009/0209794 A1, and U.S. Pat. Nos. 8,692,043 and 9,144,790, disclose processes for aromatizing lower alkanes using a particulate catalyst, where the average catalyst particle residence time in the reaction zone between regeneration treatments is in the range of about 0.1 second to about 30 minutes. Maximum ethane conversion is about 63%, but the catalyst and process conditions which achieve appreciable ethane conversion also exhibit appreciable selectivity for methane.
It is desired to produce aromatic hydrocarbon from C2+ non-aromatic hydrocarbon at greater feed conversion, particularly with less methane yield. Processes which operate at a space velocity (GHSV) greater than 1000 hr−1, are particularly desired. Reverse-flow reactors suitable for carrying out such reactions are also desired.